An important requirement for capacitors used for power leveling and ripple filtering is quick response in absorbing and releasing energy. At present, this is primarily performed by aluminum electrolytic capacitors. These are voluminous bulky components, and they fail irreparably. In other words, conventional electrolytic capacitors, such as those with a dielectric of metal oxide on a metal, are large and generally have low reliability.
Electric double layer capacitors (EDLCs), often referred to as supercapacitors or ultracapacitors, address many of the problems of conventional electrolytic capacitors. First, EDLCs can have higher capacitance per unit area compared with those conventional capacitors, often two orders of magnitude greater capacitance at the same size. EDLCs also circumvent the reliability problem common to electrolytic capacitors. The electric double layer is formed naturally at an interface when voltage is applied, and this dielectric is totally self-healing; that is, its breakdown does not mean the device is destroyed, as with conventional electrolytic capacitors. Most EDLCs are frequently used for energy storage, as well as other commercial applications where the high surface area of activated carbon allows very high capacitance. However, the small diameter and long path lengths pores result in distributed charge which limits the RC time constant to seconds and are therefore not useful for filtering applications.
Past development of EDLCs comprising vertically oriented graphene nanosheets (VOGN) on Ni substrates grown by radio frequency plasma enhanced chemical vapor deposition (RF-PECVD) has shown promising capacitive and response time resultssuitable for filtering applications (e.g., see Miller et al., “Graphene Double-Layer Capacitor with AC Line-Filtering Performance. Science, 2010, 329, 1637-1639; and Miller et al., “Vertically-Oriented Graphene Electric Double Layer Capacitor Designs”, J. Electrochem. Soc., 2015, 162, A5077-A5082). The vertical nanosheets provide a very open morphology which allows efficient ingress and egress of electrolyte corresponding to good frequency response. The density and height of sheets determines the surface area necessary to give useful specific capacitance. Although Ni (also Ta and Nb) are excellent substrates for VOGN growth because of the high solubility of carbon in those metals (which gives good ohmic contact), they are heavy and expensive.
Aluminum foil has been used for many years for electrolytic capacitors and would serve as a lighter and more affordable substrate material for fast-response, VOGN-electrode electric double layer capacitors. Unfortunately, the low solubility of carbon in aluminum and the relatively thick (2-3 nm) stable native oxide (Al2O3) covering its surface hinders VOGN growth. This causes capacitive rather than ohmic connection to the aluminum, which severely restricts the frequency response. Further, the low melting point of Al (660° C.), relative to Ni and Ta, makes it quite difficult to grow high density nanosheets by RF-PECVD.
Accordingly, there is a need for vertically oriented graphene nanosheets that can be supplied on inexpensive substrates suitable for supercapacitors.